Digital three-dimensional manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object of virtually any shape from a digital model. Three-dimensional printing is an additive process in which one or more printheads eject successive layers of material on a substrate in different shapes. Typically, ejector heads, which are similar to printheads in document printers, include an array of ejectors that are coupled to a supply of material. Ejectors within a single ejector head can be coupled to different sources of material or each ejector head can be coupled to different sources of material to enable all of the ejectors in an ejector head to eject drops of the same material. Materials that become part of the object being produced are called build materials, while materials that are used to provide structural support for object formation, but are later removed from the object are known as support materials. Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
A portion of a previously known three-dimensional object printing system 10 is shown in FIG. 4. In the view depicted in that figure, a platform 14, called a cart, is configured to ride upon track rails 22 to enable the cart to move in a process direction P between printing stations, such as the printing station 26. Printing station 26 includes four ejector heads 30 as shown in the figure, although fewer or more ejector heads can be used in a printing station. Once the cart 14 reaches the printing station 26, the cart 14 transitions to precision rails 38. Precision rails 38 are cylindrical rail sections that are manufactured within tight tolerances to help ensure accurate placement and maneuvering of the cart 14 beneath the ejector heads 30. Linear electrical motors are provided within housing 42. These motors produce electromagnetic fields that interact with a magnet 46 connected to the lower end of the cart 14 to move the cart along the track rails 22 between stations and to move the cart on the rails 38 within a station 26. Once the cart 14 is beneath the printing station 26, the printheads are operated to eject material in synchronization with the motion of the cart. Additional motors (not shown) move the printing station 26 vertically and in an X-Y plane over the cart to form an object with layers of material ejected by the printheads. Alternatively, a mechanism can be provided to move the cart 14 vertically and in the X-Y plane to enable formation of the object on the cart. Once the printing to be performed by a printing station is finished, the cart 14 is moved along the rails 22 to another printing station for further part formation or for layer curing or other processing.
An end view of the cart 14 on the rails 38 is shown in FIG. 3. At a printing station 26, bearings 34 of the cart 14 are positioned on the precision rails 38 in an arrangement that facilitates accurate positioning of the build platen on the cart 14. Specifically, a pair of bearings 34 are positioned at a right angle to one another on one of the rails 38 to remove four degrees of freedom of the cart 14, while the other bearing 34 rests on the other rail 38 to remove one more degree of freedom. As described above, linear motors in housing 42 operate to interact with a magnet positioned within housing 46 to move the cart 14 over an upper surface 50 of the housing 42. Gravity and magnetic attraction between the motors in the housing and the magnet 46 hold the bearings 34 in contact with the rails 38.
The three-dimensional additive process is performed in a printer in a layer-by-layer manner. To operate the ejectors in the printhead(s) to form a layer, a three-dimensional raster processor receives a file of three-dimensional data of the part to be produced. These three-dimensional part data can be contained in a computer-aided design (CAD) file, for example. The processor uses these data to generate a raster data file, which contains data that correspond to each layer that forms the part. A printhead driver receives the raster data file and generates pixelated data that are used to operate the ejectors in the printhead(s) for the ejection of building and support material onto a support platen to form the part layer by layer. The printhead driver and a printer controller generate signals to coordinate the movement of the platen and the printhead(s) with the operation of the ejectors in the printhead.
During the formation of a layer during part printing, an ejector in an ejector head can malfunction. A malfunctioning ejector includes one that ejects drops of material in a direction other than its intended path, ejects drops that are smaller than expected, or fails to eject material drops at all. Techniques are known for detecting malfunctioning ejectors. If the absence of material drops or the misplacement or reduction in size of the material drops is not corrected, the part can be adversely impacted enough to cause rejection of the part. Since part printing can take hours, the rejection of a part after its production substantially reduces throughput for the system. Therefore, compensating for the absence or loss of material mass at a drop location would be beneficial.